The use of semipermeable membranes for reverse osmosis or ultrafiltration processes is well known. For example, in a reverse osmosis process, high pressure saline water may be placed in contact with a semipermeable membrane which is permeable to water but relatively impermeable to salt. Concentrated brine and relatively pure water are separated thereby; the water may then be utilized for personal use such as drinking, cooking, etc.
It has now been discovered that certain membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
This ability to separate gases from a mixture stream will find many applications in commercial uses. For example, gas separation systems may be used for oxygen enrichment of air, for improved combustion efficiencies and conservation of energy resources. Likewise, nitrogen enrichment of air may be applicable where inert atmospheres are required. Other applications for oxygen enriched gases may be improving selectivity and efficiency of chemical and metallurgical processes. Similarly, inert atmospheres such as may be provided for by this invention may also be utilized in chemical and metallurgical processes. Some other applications of gas separation would include helium recovery from natural gas, hydrogen enrichment in industrial process applications, and scrubbing of acid gases. Specific uses for oxygen enrichment of air would be breathing systems for submarines and other underwater stations, improved heart-lung machines, and other lung assist devices. Another specific application of a gas separation system would be in aircraft to provide oxygen enrichment for life-support systems and nitrogen enrichment for providing an inert atmosphere for fuel systems. In addition, oxygen enrichment air can be used in furnaces for more efficient combustion; catalytic oxidations of organic compounds, i.e., mercaptans, hydrocarbons, alcohols, aldehydes, etc., to name but a few. Likewise, gas separation systems may be used for environmental benefits, e.g., methane can be separated from carbon dioxide in waste gases for sewage treatment processes and oxygen enriched air can be produced to enhance sewage digestion.
Some thin film polymers have been reported in the literature. For example, U.S. Pat. No. 3,892,665 discloses a membrane and a method for producing these membranes. In this patent, a thin polymer film is formed on the surface of a liquid, generally water, and is subsequently transferred to the surface of a porous supporting membrane. During the transfer of the thin polymer film, the porous support is maintained in a wetted stage with the liquid. In addition, the thin film can also be formed on the surface of the porous membrane if the surface of the support is first wet with the transfer liquid. This then means that the pores of the support member must be filled with liquid and, therefore, the liquid must be removed from the porous support at a period subsequent to the formation of the film in order to draw the film onto the support. In general, the thin polymer film of the reference consists of a monomolecular layer which is formed on the surface of the water wherein the individual film-forming monomer and/or polymer chains are oriented and closely packed. Subsequently, the oriented monomolecular layer or film, which is limited to a thickness in the range of from about 5 to about 25 Angstroms, is transferred to the surface of the porous support membrane. This process may be repeated until multiple monolayers are deposited on the surface of the support, the total film thickness then being from about 10 to about 200 Angstroms. Other than Van Der Vaal's forces, there is no bonding between the aggregate layers and the support. This means that the thin film of the finished membrane is weakly attached to the porous support and said membrane cannot withstand substantial back pressure when in operation. Obviously, this process is tedious and expensive and is not readily amenable to commercial use.
Another U.S. patent, namely U.S. Pat. No. 3,526,588, discloses a macromolecular fractionation process and describes a porous ultrafiltration membrane which is selective on the basis of pore size. In contradistinction to this, it is essential that the thin film membrane which is produced according to the process of the present invention is nonporous, so that gas separation operates by a diffusion-solution mechanism of transport. U.S. Pat. No. 3,767,737 which discloses a method for producing casting of "ultra-thin" polymer membranes is similar in nature to U.S. Pat. No. 3,892,665 in that the thin film of the membrane is formed on the surface of a liquid and transferred to the surface of a porous support membrane. The thin film polymer will thus inherently possess a disadvantage ascribed to the membrane of the former patent in that it cannot withstand substantial back pressure when in operation. In addition, U.S. Pat. No. 2,966,235 discloses a separation of gases by diffusion through silicone rubber which is not composited on a porous support material.
U.S. Pat. No. 4,155,793 involves a continuous method for the preparation of membranes by applying a polymer to a microporous support. However, the method of production described in this patent involves the spreading of a polymer casting solution onto the surface of a liquid substrate. The polymer which is utilized is not soluble in the liquid substrate nor is the solvent which is used compatible with the microporous support. The polymer film which constitutes the membrane is formed on the surface of the liquid and is thereafter applied to the microporous support. U.S. Pat. No. 4,132,824 discloses an ultra-thin film of a polymer composite which comprises a blend of a methylpentene polymer and an organopolysiloxane-polycarbonate interpolymer for a thickness less than about 400 Angstroms in which the interpolymer is present in an amount of up to about 100 parts by weight per 100 parts by weight of the methylpentene polymer. Likewise, U.S. Pat. No. 4,192,824 describes a method for preparing the aforementioned interpolymer by depositing on the surface of a liquid casting substrate a casting solution which comprises a mixture of methylpentene polymer and from 0 to 100 parts by weight of an organopolysiloxane-polycarbonate copolymer. The casting solution spreads over the surface of the liquid casting substrate to form a thin film following which at least a portion of the thin film is removed from the surface of the substrate. Thereafter, the film may be used in contact with a porous support as a gas separation membrane.
As hereinbefore set forth, the separation of various gases from a mixture thereof may constitute an important advance in commercial applications. This is becoming increasingly important in view of the necessity to conserve energy. A particular application would relate to increasing the thermal efficiency of combustion processes when utilizing fossil fuels in commercial combustion applications. Also, by utilizing a gas separation membrane in coal gasification, it may be possible to provide an oxygen enrichment of air for the production of low and medium British Thermal Unit (BTU) product gases as well as an oxygen enrichment of air for the combustion of these gases. For example, by placing a gas membrane separation system in close proximity to both gas production and gas combustion facilities, it would allow a site-located oxygen enrichment plant to supply both processes without the additional expense of transporting the gas or duplicating enrichment facilities.
As was hereinbefore set forth, membranes may be utilized for the oxygen enrichment of an atmosphere by facilitated transport means. The requirements for an efficient facilitating transport oxygen enrichment membrane include the characteristic of being thermally stable at ambient and above ambient temperatures, as for example, from 50.degree. to 60.degree. C.; the ability to withstand high pressures without physically destroying the membrane; being hydrolytically stable to water and/or water vapor and in a physical form which is adaptable for use as a thin film composite membrane in sheet form or as a coating to hollow fine fibers. As will hereinafter be shown in greater detail, we have now discovered that a gas enrichment membrane composite which comprises an organometallic carrier interdispersed in an interpenetrating polymer network membrane will possess all of the desirable characteristics hereinbefore enumerated.
Some prior work has been carried out in an attempt to find a gas enrichment membrane which will possess the desirable characteristics and which will possess sufficient stability to be used in gas enrichment processes for a relatively extended period of time. For example, an article in Bull. Chem. Soc. Japan, 42 (1969) examined the reversible oxygenation of Co(II) salcomines in a poly(4-vinylpyridine) matrix. However, although the oxygen carrier reacted reversibly with oxygen in the early stages, the membrane began to show irreversibility in later stages. Other articles which appeared in Biopolymers 13 and 17 utilized Co(II) and Fe(II) protoporphyrin as the oxygen carrier while the polymers include imidazole and pyridine based, poly(4-vinylpyridine), poly(N-vinyl-2-methyl imidazole), poly(4-vinylimidazole), etc. The article stated that the metalloporphyrin reacted reversibly with oxygen at subambient temperatures and showed irreversible oxidation above -20.degree. C.
Further work in gas enrichment membranes was set forth in Makromol. Chem. 178 which discussed membranes comprising Fe(II) and Co(II) porphyrins in a polymeric matrix comprising poly(N-vinylimidazole-styrene). The preparation of these systems was accomplished first by preparing a homogeneous solution of the copolymer with the metalloporphyrin and by preparing terpolymers by the copolymerization of N-vinyl imidazole, styrene and hemin dimethylester. The membrane prepared according to the first method was found to have a weak interaction between the metalloporphyrin and the polymer, the system showing no stability to oxygen in solution inasmuch as an irreversible oxidation was observed. However, in solid state, the system was stable to oxygen for a relatively long period of time at ambient temperature. The membrane prepared according to the second method was found to contain the metalloporphyrin covalently bound to the polymer. When utilized in solution, the membrane system reacted reversibly with oxygen for two or three cycles and then showed irreversible oxidation. When utilized in solid state, the terpolymer system reacted reversibly with oxygen and displayed a stability for a relatively long period of time at ambient temperature. However, both of the systems possessed a disadvantage in that the systems had a slow adsorption and desorption rate in the solid state (t.sub.1/2 .perspectiveto.20 hrs).
Another article, Makromol. Chem. Rapid Commun., 3 described a membrane comprising Co(II) doped poly(ethyleneimine) in which the membrane was prepared by coating a crosslinked membrane from linear poly(ethyleneimine) with poly(epichlorohydrin) and exposing the film to a Co(II) chloride solution. No mention was made of the stability of the membrane over repeat cycles nor was the nitrogen permeability nor the oxygen selectivity determined. Other articles have shown that liquid membranes may be prepared. For example, an article in the J. Chem. Soc., 1982 showed a liquid membrane based on Cu(I) tetraethylenepentamine as a facilitated transport oxygen enrichment membrane. While the initial oxygen selectivity was relatively high on the first cycle, it rapidly decreased to a relatively low level on the second and third cycle. Other workers in the field described, Angew. Chem. Int. Ed. Engl., 16, how to prepare synthetic oxygen carriers which were soluble in aqueous systems. Such a polymer contained a distal imidazole, hemin and a hydrophobic pocket. The oxygen carrier mimic was based on a polyurethane prepared from poly(ethylene glycol). The polymer backbone was functionalized with histidine via peptide synthetic routes with a final addition of hemin, thus mimicking a distal histidine. The proximal imidazole was added to the hemin by a similar synthetic peptide route. The synthetic oxygen carrier showed reverse binding with oxygen, however it underwent irreversible oxidation after a few cycles. Other aqueous soluble oxygen carriers were described in J. Macromol. Sci. Chem. A13 and J. Amer. Chem. Soc., 101 in which an oxygen carrier was introduced directly into the backbone of the polymer or by incorporation to a pendent group of the parent polymer. The polymers were based on copolymers of styrene and N-vinyl imidazole in one instance with the membrane showing reversible oxygen binding with half life ranging from a few minutes to one day. In the other instance, water soluble polyphosphazenes were used as carriers for metalloporphyrin. While there is a strong complex between the oxygen carrier and the polymer solution, the system was found to irreversibly oxidize both in solution and in the solid state.